Field of the Invention
The invention relates to an input amplifier with unilateral current shutoff for input signals with steep edges (in particular "high-low" edges). The input amplifier has at least one transistor with an electrode connected to an output.
CMOS input amplifiers have been known for a relatively long time and are used for the most manifold purposes in circuitry. For instance, an input amplifier of the above type is described for instance in the textbook "CMOS Analog Circuit Design" by P. E. Allen and D. R. Holberg, p. 381.
One such prior art input amplifier will be described below with reference to FIG. 3. The prior art input amplifier has a differential amplifier, one P-channel MOS transistor, and one N-channel MOS transistor. The gates of the MOS transistors and an input of the differential amplifier are connected to an input terminal which receives an input signal, and the source-to-drain path of one of the MOS transistors connects to an an operating voltage. The differential amplifier also is subject to a reference voltage.
Possible uses of those prior art input amplifiers are for instance LVTTL circuits (LVTTL stands for low voltage transistor-to-transistor logic) and SSTL logic circuits (SSTL stands for stub series terminated logic). In LVTTL logic circuits, leading and trailing voltage edges of approximately 0.8 V to 2.0 V occur, while the SSTL logic circuits have corresponding leading and trailing edges of approximately 400 mV about a reference value.
In both logic circuits, that is, the LVTTL logic circuits and the SSTL logic circuits, the trailing voltage edges are very steep, so that the input edges are correspondingly fast. If numerous input amplifiers are provided on a chip, then in a current-saving operating mode only those circuits that are triggered should actually be active.
However, it has been found that with steep edges of the input signals, a corresponding shutoff of the input amplifier is not reliably assured.
This problem has been previously attacked by employing current mirror circuits or asymmetrical input amplifiers. However, current mirror circuits consume a relatively large amount of current, while asymmetrical input amplifiers are disadvantageous in terms of the symmetry of their switching performance.